1. Field of the Invention
The present invention relates to reducing electromagnetic radiation emanating from computer enclosures, and particularly, from enclosures for relatively inexpensive, mass-produced computer systems employing printed circuit board (xe2x80x9cPCBxe2x80x9d) technology.
2. Description of Related Art
As a by-product of normal operation, electronic equipment frequently emits undesirable electromagnetic radiation, often referred to as electromagnetic interference or xe2x80x9cEMI.xe2x80x9d At the same time, equipment specifications require a maximum acceptable level of EMI outside of an electronics enclosure, to comply with government regulations and other performance requirements. For computer systems, EMI requirements are generally complied with by enclosing the computer system in an enclosure made of metal or other conductive material. Openings in the enclosure may be covered with a metallic grill or mesh screen, and the enclosure as a whole constitutes a xe2x80x9cFaraday cage.xe2x80x9d To maintain a perfect Faraday cage over a wide bandwidth, no apertures above a specified size may exist in the shield. The higher the frequency of the EMI, the smaller the shield apertures should be.
In practice, computer enclosures contain some apertures that are not covered by grills or screens, because of penetrations for connectors, openings for insertions of items such as floppy disks, and assembly joints, among other things. Despite openings such as these, EMI requirements for most computer systems have been easily complied with in the past using relatively simple, low-cost enclosures. But the advent of modern computer systems has made compliance with EMI requirements more difficult, particularly for lower-end computer systems that are sold in an extremely cost-competitive market.
That is, it is generally more difficult to meet EMI requirements for modern systems without undesirable cost increases in the enclosures or other system components. Various changes in the industry underlie these new difficulties. For one thing, the increasingly high frequency of commonly available microprocessors, especially above about 500 megahertz, means that computer enclosures must be better sealed against transmission of RF frequencies. The higher-frequency microprocessors emit EMI at higher frequencies, which, in turn, can emanate from an enclosure through smaller openings than EMI at lower frequencies. And as frequencies and edge rates in high-speed digital designs continue to increase, EMI as a result of radiation through slots, apertures, and seams in shielding enclosures is becoming increasingly problematic. There is sufficient energy at low-order clock harmonics to cause EMI problems above a few hundred MHz as a result of exciting cavity modes of the enclosure, and efficiently driving even small length slots and apertures that are unavoidable in a practical design.
Consolidation among manufacturers, and the drive towards cost reduction in the computer industry in general, also plays a role. To build a computer system at a competitive cost, a computer manufacturer typically will include certain components that are only available as stock items from a limited number of manufacturers. Peripheral DVD drives are an example of a typical stock item made by relatively few manufacturers. The computer manufacturer often has no direct control over the particular configuration of these stock items. Consequently, a particular stock component that is otherwise desirable may provide a pathway for EMI, particularly at high frequencies, to radiate from a computer enclosure. This pathway may be blocked by customization of the component, but customization of a stock item can add substantially to the system cost.
In particular, certain peripheral components, such as CD and/or DVD drives, essentially include a rectangular-tubular metallic enclosure, very much like a rectangular cross-section waveguide, that is not shielded from transmission of EMI at one or both ends, and are generally mounted. inside computer system enclosures with one unshielded end passing through the computer enclosure. For example, in disk drives such as CD, DVD, and floppy disk drives, the disk door on the exterior of the computer enclosure is frequently made of a nonconductive plastic material that does not block transmission of EMI. At the same time, EMI may enter the opposite end of the peripheral enclosure via a cable connection or opening. Hence, the peripheral enclosure can form an efficient waveguide for transmission of EMI at certain frequencies to the exterior of the computer enclosure. For example, certain modern CD/DVD peripherals provide an efficient waveguide for transmission of EMI at frequencies in the range of about 800-1000 MHz. Other components and compartments within computer enclosures may also act as waveguides at these and other frequencies, depending on the details of the component and its relationship to other components of the computer system.
One approach for eliminating the waveguide effect of peripherals such as disk drives is to construct the opening door of a metallic or conductive material. This approach is likely to add to the cost of the component, and is not effective when the peripheral door is open. Another approach is to employ multiple ground points for the peripheral component, but this approach may increase assembly cost, and multiple ground points are subject to being disrupted during repair or replacement of the peripheral component.
A computer enclosure may contain other EMI openings which, for one reason or another, are difficult to block in a reliable, relatively permanent, and low-cost manner. It is desired, therefore, to provide an alternative method and apparatus for preventing EMI transmission through such openings in computer enclosures, including but not limited to openings created by disk drive peripheral components, that overcomes the limitations of the prior art.
The present invention provides a method and apparatus for reducing electromagnetic radiation emanating from a computer enclosure, that overcomes the limitations of the prior art. The method and apparatus may be adapted for application to a computer enclosure enclosing at least one source of electromagnetic radiation of a computer system, where the enclosure includes an unshielded opening in the computer enclosure through which an undesirable amount of electromagnetic radiation from the source is capable of passing to an exterior of the computer enclosure.
Generally, the apparatus includes a system board mounted to a frame of the computer enclosure, and a riser card connected to the system board within the computer enclosure. The riser card is a PCB having at least one ground layer (i.e., a signal return layer) and a signal layer, and extends transversely from the system board. The riser card divides an interior space of the computer enclosure into two compartments, a source compartment containing the source of electromagnetic radiation, and an unshielded compartment having the unshielded opening in it. The ground layer of the riser card is positioned towards a side of the riser card facing the unshielded compartment. The apparatus further includes a sheet metal barrier attached to the frame of the computer enclosure and around a perimeter of the ground layer. The riser card and the sheet metal barrier together form an EMI shield between the source compartment and the unshielded compartment, whereby the undesirable amount of electromagnetic radiation from the source is prevented from passing to an exterior of the computer enclosure. Compartmentalization by the riser card may be most conveniently accomplished in computer systems constructed according to the NLX form factor, in which the prior art frame components and riser card provide a partial but nearly complete physical barrier between two internal compartments of the system.
In some configurations, the riser card may have traces on a side facing the source compartment. These traces may act as antennae for receiving electromagnetic radiation from the source compartment and conducting it to a cable socket on the side of the riser card facing the unshielded compartment. A cable connected to the socket may then transmit the radiation to a device such as a CD/DVD drive having an unshielded opening to the exterior of the enclosure. Accordingly, in an embodiment of the invention, the traces on the source side of the riser card are covered by interposing a sheet metal shield between the riser card and the source compartment. The sheet metal shield may be attached to the riser card and connected to its ground layer by fasteners, while being spaced a short distance apart from it by a plurality of spacers. The shield prevents the traces from receiving electromagnetic radiation from the source compartment.
In an alternate embodiment, the riser card itself is configured to prevent the traces from receiving radiation from the source compartment. A second ground layer is provided in a layer of the riser card, interposed between the at least one signal layer and the source compartment. The riser card may contain six or more layers. The second ground layer covers the traces and thereby prevents them from receiving source radiation.
A more complete understanding of a method and apparatus for reducing electromagnetic radiation emanating from a computer enclosure will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the invention. Reference will be made to the appended sheets of drawings which will first be described briefly.